1. Field of the Invention The present invention relates to a process for preparing acryl modified polyester resins for use in powder coatings and also a powder coating composition containing said acryl modified polyester resins. In particular, the present invention provides a process for preparing acryl modified polyester resins by using good characteristics of conventional polyester resins and acryl resins and a powder coating composition by using a triglycidyl isocyanurate (TGIC) as a curing agent.
2. Description of the Prior Art
There are many conventional resins for powder coatings which could be selected in accordance with their uses. When a resistance against weather is required, carboxylated polyesters and the TGIC have been used. In this case, mechanical strength and an adhesive performance are good but stain and chemical resistance are relatively poor when compared with powder coatings which are composed of epoxy resins and poly carboxylic acids or composed of the carboxylated polyester and epoxy resins. Above prior art coatings which use the carboxylated polyesters and the TGIC may also have disadvantages as follows; a) it is difficult to form a smooth surface thereof because of the insufficient flow of the melt, b) poor appearance of the final coated film, and c) the coatings appear to have an orange peel and pin hole phenomena since when they are coated and cured air can not be removed sufficiently. Further, the prior art coatings can improve resistance against weather better than the powder coatings which use the epoxy resins, but cannot provide the good resistance against weather compared with a solution typed acryl coatings or a powder typed acryl coatings.
Therefore, the conventional resins are unsuitable for use in the fields which require good resistance against weather. The conventional acryl typed resin coatings show weather and chemical resistance but require a high cure temperature such as at least above 180.degree. C. A low curing temperature degrades resistance against weather, solvent resistance, a resistance against stain, thermo-resistance and degrades mechanical properties as well due to the insufficient cure. Furthermore, the solvent typed coatings useful for decorative coatings could be prepared into a thin coating having a thickness of 25 to 50 microns, but conventional powder coatings are difficult to make the thin coating. Also, when the solution typed coatings are heated and melted, a low flowage resulting from a high melting viscosity may cause an appearance thereof to be poor.
If one arbitrarily lowers curability in order to improve the flowage, it may be possible to prepare the good appearance but mechanical properties and solvent resistance are degraded. Moreover, when one also prepares a thin coating by means of lowering a viscosity and lowering a softening point, the storage stability of coatings should be degraded due to the blocking thereof during the storage.